Anything at all from Freeverse- Bridge, Spades, and Hearts, all networked for online play; gin rummy, cribbage, and a nasty little piece of work called Enigma... all available at http://www.freeverse.com

And a true mac classic, for those who feel like bashing a few orcs or trolls-Realmz. While a few of the scenarios are (very) problematic under 9.x, many still work (I'd recommend Trouble and War in the Swordlands). They also have quite a number of other games. They're available at: http://www.fantasoft.com